On Fri, Jun 10, 2016 at 4:14 PM, Robert Haas <robertmh...@gmail.com> wrote:
> Although I have done a bit of review of this patch, it needs more
> thought than I have so far had time to give it.  I will update again
> by Tuesday.

I've reviewed this a bit further and have discovered an infelicity.
The following is all with the patch applied.

By itself, this join can be pushed down:

contrib_regression=# EXPLAIN SELECT 13 FROM ft1 RIGHT JOIN ft2 ON
ft1.c1 = ft2.c1;
                      QUERY PLAN
 Foreign Scan  (cost=100.00..137.66 rows=822 width=4)
   Relations: (public.ft2) LEFT JOIN (public.ft1)
(2 rows)

That's great.  However, when that query is used as the outer rel of a
left join, it can't:

contrib_regression=# explain verbose select * from ft4 LEFT JOIN
(SELECT 13 FROM ft1 RIGHT JOIN ft2 ON ft1.c1 = ft2.c1) q on true;
                                         QUERY PLAN
 Nested Loop Left Join  (cost=353.50..920.77 rows=41100 width=19)
   Output: ft4.c1, ft4.c2, ft4.c3, (13)
   ->  Foreign Scan on public.ft4  (cost=100.00..102.50 rows=50 width=15)
         Output: ft4.c1, ft4.c2, ft4.c3
         Remote SQL: SELECT c1, c2, c3 FROM "S 1"."T 3"
   ->  Materialize  (cost=253.50..306.57 rows=822 width=4)
         Output: (13)
         ->  Hash Left Join  (cost=253.50..302.46 rows=822 width=4)
               Output: 13
               Hash Cond: (ft2.c1 = ft1.c1)
               ->  Foreign Scan on public.ft2  (cost=100.00..137.66
rows=822 width=4)
                     Output: ft2.c1
                     Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
               ->  Hash  (cost=141.00..141.00 rows=1000 width=4)
                     Output: ft1.c1
                     ->  Foreign Scan on public.ft1
(cost=100.00..141.00 rows=1000 width=4)
                           Output: ft1.c1
                           Remote SQL: SELECT "C 1" FROM "S 1"."T 1"
(18 rows)

Of course, because of the PlaceHolderVar, there's no way to push down
the ft1-ft2-ft4 join to the remote side.  But we could still push down
the ft1-ft2 join and then locally perform the join between the result
and ft4.  However, the proposed fix doesn't allow that, because
ph_eval_at is (b 4 5) and relids for the ft1-ft2 join is also (b 4 5),
and so the bms_is_subset(phinfo->ph_eval_at, relids) test returns

Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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